In this modern era, organizations greatly rely on computer networks to share information throughout the organization in an efficient and productive manner. Organizational computer networks are now becoming large and ubiquitous. Assuming that each staff member has a dedicated workstation, a large scale company would have few thousands workstations and many server on the network. It is likely that these workstations may not be centrally managed, nor would they have perimeter protection. They may have a variety of operating systems, hardware, software, and protocols, with different level of cyber awareness among users. Now imagine, these thousands of workstations on company network are directly connected to the Internet. This sort of unsecured network becomes a target for an attack which holds valuable information and displays vulnerabilities. In this chapter, we describe the major vulnerabilities of the network and significance of network security. In subsequent chapters, we will discuss the methods to achieve the same. Physical Network A network is defined as two or more computing devices connected together for sharing resources efficiently. Further, connecting two or more networks together is known as internetworking. Thus, the Internet is just an internetwork – a collection of interconnected networks. For setting up its internal network, an organization has various options. It can use a wired network or a wireless network to connect all workstations. Nowadays, organizations are mostly using a combination of both wired and wireless networks. Wired & Wireless Networks In a wired network, devices are connected to each other using cables. Typically, wired networks are based on Ethernet protocol where devices are connected using the Unshielded Twisted Pair (UTP) cables to the different switches. These switches are further connected to the network router for accessing the Internet. In wireless network, the device is connected to an access point through radio transmissions. The access points are further connected through cables to switch/router for external network access. Wireless networks have gained popularity due to the mobility offered by them. Mobile devices need not be tied to a cable and can roam freely within the wireless network range. This ensures efficient information sharing and boosts productivity. Vulnerabilities & Attacks The common vulnerability that exists in both wired and wireless networks is an “unauthorized access” to a network. An attacker can connect his device to a network though unsecure hub/switch port. In this regard, wireless network are considered less secure than wired network, because wireless network can be easily accessed without any physical connection. After accessing, an attacker can exploit this vulnerability to launch attacks such as − Sniffing the packet data to steal valuable information. Denial of service to legitimate users on a network by flooding the network medium with spurious packets. Spoofing physical identities (MAC) of legitimate hosts and then stealing data or further launching a ‘man-in-the-middle’ attack. Network Protocol Network Protocol is a set of rules that govern communications between devices connected on a network. They include mechanisms for making connections, as well as formatting rules for data packaging for messages sent and received. Several computer network protocols have been developed each designed for specific purposes. The popular and widely used protocols are TCP/IP with associated higher- and lower-level protocols.